Method of obtaining selective directional critical elongation in sheet magnetic material



a a w w w. MM ma 9-H H 5 A m k C F H m w C. E. MERENESS Filed May 25, 1957 ELONGATION IN SHEET MAGNETIC MATERIAL Jan. 24, 1961 METHOD OF OBTAINING SELECTIVE DIRE nited Charles E. Mereness, Fort Wayne, Ind., nssignor to General Electric Company, a corporation of New York Filed May 23, 1957, Ser. No. 661,144

2 Claims. (Cl. 29-1555) This invention relates to processes for improving the directional magnetic qualities of magnetic materials where the material is subjected to heat treatment subsequent to undergoing a critical elongation, and more particularly to an improved method for obtaining selective directional critical elongation in sheet magnetic material.

One of the most important factors affecting the magnetic properties of magnetic materials-and particularly relatively soft magnetic materials such as are commonly used to form the magnetic cores for dynamoelectric machines and stationary induction apparatus-is the orientetion of the crystal lattice of the material with respect to the direction in which flux is to travel through the material when it is incorporated in a magnetic device. For instance, in the case of iron, which has a body centered cubic type of lattice structure, it has been shown that if the cubes are aligned so that the flux travels in the same direction as an edge of the cube, the least possible resistance to the passage of the magnetic lines of flux results. If the cube is positioned so that the lines of flux must pass through a diagonal of the cube, magnetization becomes quite difficult. An intermediate situation occurs where the cube is positioned so that the magnetic lines of flux travel diagonally across a face of the cube.

In recognition of the desirability of providing proper orientation of the crystal lattice, particularly in iron and iron alloys, suitable processes for obtaining the improved orientation have been developed. While there are many variations between the different processes, practically all incorporate the imparting of a critical strain (generally a 2 to 20 percent elongation, with the optimum figure deending on the particular material) and then heat treating for a suitable time at a suitable temperature. In the past, the required strain has been imparted to the material by continuing processes such as rolling, where the material elongation all occurs in substantially the same direction. In general, the cube edges of the crystal lattice Will be found to run parallel to the direction of elongation. Thus, in the continuous processes such as rolling, the improved magnetic properties will be fully found in iron and iron alloys only if the intended direction of magnetic flux is parallel to the direction of rolling. In order to make efiicient use of the best magnetic path in the material, a flux conductor-or magnetic core as it is called-made from rolled material must necessarily be segmented so that in each segment the flux will be traveling in the desired direction; in such a case, the utilization of separate segments to make an integral magnetic core presents the problem of overcoming the increased resistance to flux travel which occurs at the faces of abutting core segments. However, in many types of equipment, such as small dynamoclectric machines, the magnetic core is made up of a stack of laminations where each lamination constitutes the entire cross section of the core. The fiux must travel in many different directions within such a single lamination and segmentation of the core is impractical. Consequently, in order to extend atent C "ice the advantages inherent in the optimum use of magnetic material to apparatus such as small dynamoelectric machines, etc., it is desirable to provide a process whereby magnetic paths of low reluctance may be provided in any desired direction. With such a process, the flux can always follow the path of low reluctance even though different directions of travel are required in different parts of a single lamination. Since the direction of the critical elongation elfects the direction of the optimum magnetic path, it is therefore necessary to effect a system of quickly and economically providing the proper elongation, so that upon annealing the desired low reluctance magnetic paths will be provided in the lamination.

It is therefore an object of this invention to provide an improved method of obtaining selective directional critical elongation in sheet magnetic material.

More particularly, it is an object of this invention to provide a method of obtaining critical elongation in sheet magnetic material where the elongation is desired in a plurality of directions in a plurality of locations in the same sheet.

A further object of the invention is to make such a process readily and economically applicable to high production methods.

The invention provides a method of obtaining selective directional critical elongation in sheet magnetic material which is intended to have magnetic flux paths in one or more directions provided therein. In one aspect thereof, the method comprises moving a die substantially normal to the plane of the sheet until it cooperates with the part supporting the sheet to engage it with a predetermined compressive force. Either the die member, the support member or both are formed to cause metal flow in the sheet which is primarily restricted to the directions which the flux paths will take when the sheet material is put to its intended use.

By providing the required critical strain by a substantially normally moving die rather than by the usual rolling or stretching methods, it is possible to form the face of the die so that any desired elongation may quickly and economically be provided at any given point on the face of the sheet.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing.

In the drawing:

Figure 1 is a view in perspective of apparatus illustrating the basic feature of this invention;

Figure 2 is a view in perspective of an application of the inventive method to a lamination for a dynamoelectric machine core member; and

Figure 3 is a photographic reproduction showing the directional grain growth induced by use of the improved process.

Referring now to Figure l of the drawing, there is shown a piece 1 of sheet magnetic material (normally iron or an iron alloy) which is seated on a flat base 2, as shown. A die 3 is movable in a direction normal to the plane of sheet 1 from the position shown in phantom to the position shown in solid outline, as illustrated by the arrow A. The surface 4 of the die is suitably contoured to cause metal flow in sheet 1 in the predetermined desired direction. Assuming that the metal flow is desired in the direction shown by arrow B, the optimum metal flow in that direction may be obtained by contouring surface 4, as shown, in the shape of a substantially cylindrical surface with the elements thereof approximately normal both to the direction of motion of the die and to the desired optimum magnetic direction. With such a contour for the die surface, the metal of sheet 1 is caused to flow in direction B as the sheet is compressed between die 3 and the support 2. A strain gradient results, with the maximum strain occurring at the point 5 of deepest penetration of the die in sheet 1 and decreasing gradually to zero at point 6. Provided care is exercised so that the strain at point 5 does not exceed the maximum critical elongation (on the order of 20 percent), the critical strain will have been imparted to sheet 1 at all points extending in both directions from point 5 to a point 7 where the minimum critical strain (on the order of about 2 percent) is provided. It is now necessary merely to apply the appropriate heating step for the desired directional magnetic qualities to be obtained in the direction B between points 7-7 in sheet 1.

To provide a specific illustration of the technique described in connection with Figure 1, a ring shaped lamination formed of 2.56% silicon steel with a thickness of approximately .027 inch was subjected to compressive force substantially in the manner shown in Figure 1. A die was provided with a surface contoured in the form of a cylinder having a two inch radius, with the element at the center of the cylindrical surface extending radially with respect to the lamination. The die weighed thirty-eight pounds, and the necessary compressive force was provided by impact through letting it drop in free fall from a height of 1.75 inches. A slight overlap of the depressions resulting from the impact blows was obtained by rotating the ring shaped lamination around its center, a circumferential distance of /s inch after each impact blow and an average elongation on the order of 3 to 4 percent resulted. Reference to Figure 3 shows that a considerable degree of grain growth in the circumferential direction was obtained, thus effecting a marked improvement in the magnetic qualities of the lamination in the circumferential direction.

Referring now to Figure 2, it can be seen how the concept explained in Figure 1 can be utilized to obtain selective directional critical elongation. A standard electric motor lamination 9, formed in the usual manner from a sheet of magnetic material, has a yoke portion 10 and a plurality of teeth 11 formed between slots 12. It is intended that the flux travel in a circumferential direction in the yoke portion, as shown by the arrow C, and in a radial direction in the teeth, as shown by arrow D. In order to achieve this effect, it is necessary to obtain the critical elongation in a circumferential direction in the yoke 10 and in a radial direction in the teeth 11, and either the support on the moving part of the die or both must be provided with a suitably contoured surface. In Figure 2, the stationary supporting part 13 has been provided with the contoured surface (as opposed to the moving die 3 in Figure 1, to show that either can be so provided). With lamination 10 supported on part 13, moving die part 14 is then lowered in a direction substantially normal to the plane of lamination 10 with a predetermined compressive force. The portion of support 13 which lies under lamination yoke 10 consists of a series of surfaces 15 in the form of frustoconical segments each extending in a substantially radial direction, i.e., normal to the circumferential direction. Beneath the teeth 11 of lamination 9, die support 13 has a series of circumferentially extending surfaces 16 in the form of segments of toroids. When part 14 is lowered to compress the lamination 9 between parts 13 and 14, the proper degree of metal flow to achieve the critical elongation will be provided in each part of the lamination in the manner which is completely described in connection with Figure 1.

It can be seen from the foregoing that the invention provides an improved method of effecting selective critical elongation in one lamination.

It will, of course, be understood that while various curved surfaces have been mentioned as suitable, other surfaces to effect the desired purpose will readily come to mind upon disclosure of the basic invention. Therefore, while the invention has been explained by describ ing a particular embodiment thereof, it will be apparent that improvements and modifications may be made without departing from the scope of the invention as defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of obtaining selective directional critical elongation in a sheet of magnetic material having a circumferential yoke portion and radially extending tooth portions comprising the steps of placing the lamination on a support member, moving a die member substantially normal to the plane of said sheet while holding the sheet stationary until said die member engages said sheet with a predetermined compressive force, said die member being'formed with a first portion having a substantial plurality of radially extending frustoconical surface segments engaging said circumferential yoke portion of the sheet and being formed with a second portion having a plurality of concentric circumferentially extending surfaces in the form of toroidal segments, said frustoconical surface segments being circumferentially spaced from each other so that said predetermined compressive force causes the underlying part of the circumferential yoke portion between each of said frustoconical surface segments to undergo circumferential critical elongation and said toroidal segments being axially spaced from each other to provide a critical elongation in the underlying part of the tooth portion between each of said toroidal segments when said predetermined compressive force is applied to said sheet.

2. The method of obtaining selective directional critical elongation in a motor lamination of magnetic material having a circumferential yoke portion and radially extending tooth portions forming slots therebetween comprising placing the lamination on a first member, moving a second member substantially normal to the plane of said lamination while said lamination is stationary to apply a predetermined compressive force to said lamination, at least one of said members being formed with a portion having a substantial plurality of radially extending frustoconical surface segments engaging said circumferential yoke portion of said lamination and being formed with a portion having a plurality of concentric circumferentially extending surfaces in the form of toroidal segments engaging said radially extending tooth portions, said radially extending frustoconical surface se ments being spaced from each other so that when said predetermined compressive force is applied to said lamination the underlying part of the yoke portion between each of said frustoconical surface segments is critically elongated in a circumferential direction and said circumferentially extending toroidal segments being radially spaced from each other so that when said predetermined compressive force is applied to said lamination, the underlying part of the tooth portion between each of said toroidal segments is critically elongated in a radial direction.

References Cited in the file of this patent UNITED STATES PATENTS 1,898,061 Otte Feb. 21, 1933 2,053,l62 Pfalzgraff Sept. 1, 1936 2,489,977 Porter Nov. 29, 1949 2,700,006 Dunn Jan. 18, 1955 2,792,511 Horstman May 14, 1957 

